The most common forms of chemical sterilants used in hospitals and industrial sterilization facilities for sterilizing various plastic, fabric, paper, glass and metal articles consist of 100% ethylene oxide or a blend of 12% ethylene oxide and 88% by weight of dichlorodifluoromethane gas (e.g. "Freon 12" gas), the latter being an inert gas which reduces the flammability of ethylene oxide. There are concerns related to dumping of chemical sterilants such as ethylene oxide into the environment. Clouds of sterilant may drift into inhabited areas resulting in persons being exposed to toxic ethylene oxide. Additional concern has been expressed about the discharge of dichlorodifluoromethane gas into the environment as to its effect on the ozone layer of the upper atmosphere.
Current technology, as represented by U.S. Pat. No. 3,549,312, for recovering ethylene oxide sterilant gas consists of evacuating the sterilizing chamber and feeding the sterilant into a compressor and condenser to liquefy and collect the sterilant mixture in a holding tank. In this process, a large amount of the sterilant gas (approximately 10 to 15%) remains in the sterilizing chamber following evacuation. The water seal vacuum pumps typically employed on sterilizers are only capable of drawing vacuums from 1 to 2 pounds per square inch absolute (psia). Additionally, some items being sterilized, such as catheters and pouches, cannot withstand deep vacuums. Other types of high vacuum pumps are not used because the large quantities of moisture in the sterilizing processes contaminate the vacuum pumps and the vacuum pump seal oil. This can damage the pump, resulting in costly repairs and unreliable operation. Vacuum pump oil will also contaminate the sterilant being collected. For medical devices, this is unacceptable. The sterilant has to be kept pure.
In order to remove the remaining sterilant from a sterilizing chamber following the first evacuation, air is often pulsed into the chamber and the chamber is evacuated in a series of air pulses and vacuum steps to dilute the sterilant from the sterilized materials and the sterilizing chamber. The dilution with air makes it impractical to collect the remaining sterilant after the first vacuum which is drawn on the sterilizing chamber. The rest of the sterilant is generally discharged into the environment. In many locations throughout the world, however, government regulations prohibit such discharges. New York State, for instance, requires 99% reduction of the sterilant emission or the best available technology. To comply with allowable discharge limits, it is necessary to feed the discharge stream into a chemical scrubber which will remove the ethylene oxide. The Freon gas is still discharged into the environment with scrubber systems. Such discharges are not allowed in some locations in Europe, however, and it is expected that they will be prohibited in the United States in the future. The result is that even with very expensive recovery equipment the current technology does not solve the emission control problems.
There is additional concern about the potential of creating explosive conditions when air is used to dilute the ethylene oxide sterilant from the sterilized load and the sterilizing chamber. When air is pulsed into the sterilizing chamber, the ethylene oxide remaining in the load is compressed to a higher concentration locally within the load and mixed with air. This produces a hazardous condition because ethylene oxide is explosive when mixed with air. Thus, purging the system with air is not a good practice. Additionally, it is unsafe to attempt recovery of pure ethylene oxide from the mixture with air which would be discharged from the sterilizer.
Other non-condensing gaseous dilutents such as nitrogen and carbon dioxide have been used to flush sterilant from the sterilizing chamber in order to prevent the formation of explosive mixtures. As with air, the partial pressure of the sterilant in these mixtures of non-condensing dilutent gas and sterilant requires extremely cold temperatures in the condenser and high pressure compressor systems to collect the sterilant. The inefficiency of such systems makes it impractical to collect more than the sterilant discharged during the first vacuum drawn on the sterilizing chamber following sterilization.
The ideal way to handle chemicals which pollute the environment is to use the chemicals for the intended purpose and then recover them in a form that can be reused or converted into another useful product. Considering current and future discharge limits for ethylene oxide and Freon gas, an acceptable process must capture virtually all of the sterilant for reuse, conversion into another useful product or conversion into a non-polluting chemical form. Additionally, the sterilant must be removed from the sterilizing chamber without creating a hazardous condition in either the sterilizing chamber or the collection apparatus.
It is an object of the present invention to provide a method for capturing virtually all of the sterilant gas from sterilizing chambers such that the sterilant can be reused or converted to a useful by-product or to a non-polluting compound.